For a satellite positioning system receiver, a duration of radionavigation signal acquisition which is short constitutes a sought-after characteristic for a positioning system. Specifically, when turning on a receiver, the duration of acquisition is the duration for which the positioning information cannot yet be delivered by the receiver.
The manner of operation of the GPS system is succinctly recalled. It consists of a constellation of 28 satellites and of a terrestrial network of reference stations on land. Each satellite gravitates at about 20,000 km from the Earth with a period of revolution of 12 hours. Each of them transmits two signals, one at 1575.452 MHz for civil applications and the other at 1227.6 MHz for reserved-access applications. The signal transmitted by a satellite consists of a carrier, optionally of a sub-carrier in the case of a BOC modulation, modulated by a known spreading code and optionally by unknown data. All the satellites transmit on the same frequencies and the signals transmitted are differentiated by their code.
These codes generally exhibit a period T, which may be short, for example 1 ms, or very long on the time scale considered, for example a week, but they may also be non-periodic, as is the case for example for encrypted signals. The codes typically consist of a large number of elementary time divisions, also called code “chips” which have a mean duration equal to Dchip.
The positioning of the receiver is obtained by measuring the distance between a satellite and the receiver on the basis of the duration of propagation of the signal between this satellite and the receiver. In the receiver, a replica of the code transmitted is generated locally; the time shift between the signal received and the local signal, that is to say the replica of the code, corresponds to the sought-after duration of propagation. This shift is measured by placing the signal received and the local signal in phase; the criterion of placing in phase corresponds to maximizing the correlation function of the two signals, that is to say to searching for a peak in correlation results between the signal received and the local signal, assumptions of different shift between the signal received and the local signal being considered for each correlation calculation.
The correlation calculations are performed on the basis of the real and imaginary components of the signal received, resulting from a sampling of the analog radionavigation signal performed at a frequency Fe of greater than 2/Dchip, where Dchip is the mean duration of a code chip, according to Shannon's criterion. At the output of the antenna of the receiver, the signal is, in a conventional manner, converted into intermediate frequency, filtered, sampled, then converted into baseband by digital processing, before correlation with a local code of a satellite.
A correlation calculation is based on an assumption made about the date of receipt of the signal transmitted by the satellite at the receiver antenna level. Correlation calculations are performed for various assumptions corresponding to various reception dates spaced apart by a duration of half a code chip with respect to one another. The correlation calculations are performed over an integration interval whose duration Tint can be varied as a function of the signal-to-noise ratio predicted a priori. For a periodic code of period T equal to 1024 chips, this makes it necessary to test up to (2.1024)=2048 assumptions, i.e. consequently 2048 correlation calculations to be carried out.
Additionally, in this case, the calculation of a correlation between the signal received and the local signal for an assumption regarding the date of receipt of the code received corresponds to Tint·Fe products between samples of the two signals followed by Tint·Fe−1 sums of the results of the products. When the duration of a calculation of a correlation equals DCalcul, and if the calculations of the 2048 correlations are carried out sequentially, the total duration of the calculation of the correlations then equals 2048·DCalcul. This total duration can exceed the ten or so minutes for placing in phase the code of the signal received, that is to say for accessing and using the data produced by the satellite which transmits the signal.
With integration interval of fixed duration, a first solution for reducing the total duration of the calculation of the correlations consists in reducing the duration of a correlation calculation, for example by performing the operations (products followed by sums) in parallel rather than performing them in series as described previously. In this way, the total duration of calculation of the correlations is reduced, since the operations are carried out simultaneously.
The standpoint of this solution is adopted hereinafter.
In a certain number of situations, the reduction obtained in the total duration of the calculation of the correlations by the first solution presented is not sufficient, this being the case for example when the period T of the code is long or when the number of elementary correlations to be performed is multiplied because of a significant number of assumptions to be made about the frequency of the signal to be considered in order to compensate for the Doppler effect.
The aim of the invention is therefore to be able to reduce the total duration of the calculation of the correlations by avoiding repeating intermediate calculations which are common from one correlation calculation to another.